Chen Weijie

Changing the alkyl chain position of a small‐molecule donor provides optimized conformation, improved phase aggregation, and enhanced photovoltaic properties. The strategy affords 12.3% efficiency single‐junction all‐small‐molecule organic solar cells (ASM OSCs) with reduced recombination and enhanced carrier lifetimes. The power conversion efficiency of 12.3% is higher than all reported single‐junction ASM OSCs.

Molecular stacking plays an important role in defining the active layer morphology in all‐small‐molecule organic solar cells (ASM OSCs). However, the precise control of donor/acceptor stacking to afford optimal phase separation remains challenging. Herein, the molecular stacking of a small‐molecule donor is tuned by changing the alky chain position to match a high‐performance small‐molecule nonfullerene acceptor (NFA), Y6. The alky chain engineering not only affects the planarity of the small‐molecule donor, but also the molecular aggregation and the active layer morphology, and thus the photovoltaic performance. Notably, single‐junction ASM OSCs with 12.3% power conversion efficiency (PCE) are achieved. The PCE of 12.3% is among the top efficiencies of single‐junction ASM OSCs reported in the literature to date. The results highlight the importance of fine‐tuning the molecular structure to achieve high‐performance ASM OSCs.

New natriumion‐functionalized carbon nano‐dots (CNDs@Na) are rationally designed for planar inverted perovskite solar cells as an interface modification layer to reduce interfacial defects. CNDs@Na interfacial modification passivates surface trap states and reduces trap density at the interface, which facilitates photogenerated holes extraction and suppresses charge recombination.

Realizing the full potential of perovskite photovoltaic requires stringent control over nonradiative losses in the devices. Herein, the interfacial carrier recombination of inverted planar perovskite solar cells (PSCs) is suppressed using rationally designed natriumion‐functionalized carbon nano‐dots (CNDs@Na). The binding effect of carbon dots on Na⁺ inhibits the interstitial occupancy of alkali cations and reduces the microstrain of the polycrystalline film. Furthermore, modified surface wettability improves the ordering and crystal size of perovskite, which restrains ion diffusion and improves interfacial contact, leading to reduced interfacial charge recombination. Consequently, the effective interfacial passivation and crystallization control enhance the photovoltaic performance and long‐term stability of PSCs, resulting in an efficiency of over 20% with negligible hysteresis.

A small‐molecule acceptor (IDTS‐4F) is designed for a ternary approach, which enables the simultaneous increase in open‐circuit voltage and short‐circuit current density without sacrificing fill factor. The two acceptors form homogeneous acceptor phases, which synergize them with the increase in phase purity and crystallinity and the reduction in domain size, whereas the charge mobilities and recombinations are maintained.

Herein, an A–D–A‐type nonfullerene acceptor (named IDTS‐4F) with an alkyl thiophenyl side chain and dimethoxy thiophene bridging unit is reported. The use of an alkyl thiophenyl group is important, as the insertion of sulfur atoms can slightly downshift the highest occupied molecular orbital (HOMO) level of the molecule and allows IDTS‐4F to match with state‐of‐the‐art donor polymer PM6 (or PM7). Compared with conventional nonfullerene acceptors, IT‐4F, the IDTS‐4F molecule, has a smaller optical bandgap and higher lowest unoccupied molecular orbital (LUMO) level, which are beneficial to increase the V _oc and J _sc of the devices. Nonfullerene organic solar cell devices are fabricated using IDTS‐4F. Although the binary device based on IDTS‐4F exhibits a lower fill factor (FF, 70%), the ternary device by incorporating 0.2 of IDTS‐4F and 0.8 of IT‐4F (with PM6 as the donor polymer) can simultaneously achieve a higher V _oc and J _sc, while maintaining the high FF (77%) of IT‐4F based system. Morphology characterizations indicate the formation of homogeneous film morphology, the large increase in phase purity and crystallinity, and the reduction in domain size upon addition of crystalline IDTS‐4F, while the electron/hole mobilities and recombination losses of the IT‐4F system are both maintained.

The power conversion efficiency of N2200‐based all‐polymer solar cells (all‐PSCs) can be drastically enhanced from ≈1% to ≈11% by simply changing the solvent from chlorobenzene and 2‐methyltetrahydrofuran (Me‐THF). In‐depth investigations reveal that the preaggregation of donor (PTzBI) and acceptor (N2200) polymers in 2‐Me‐THF is the key to enable such high performance for N2200‐based all‐PSC device.

Herein, all‐polymer solar cells (all‐PSCs) are studied based on PTzBI:N2200 system processed from two different solvents, chlorobenzene (CB) and 2‐methyltetrahydrofuran (Me‐THF). It is found that the preaggregation of the donor and acceptor polymers in Me‐THF is the key factor that enables a drastic enhancement in cell efficiency from ≈1% (processed by CB) to ≈11% (processed by Me‐THF). When using CB as the solvent, both donor and acceptor polymers are well dissolved and mostly disaggregated. In contrast, the donor and acceptor polymers both exhibit strong aggregation in Me‐THF. As a result, the donor and acceptor blend films processed from Me‐THF exhibit pure domains with appropriate molecular packing structure, which leads to high charge mobilities (10⁻³–10⁻⁴ cm² V⁻¹ s⁻¹) and fill factors (FFs; 75%), whereas the blend films processed by CB suffer from highly miscible and impure domains, hence decreasing the charge mobilities by 1–2 orders of magnitude compared with those of the corresponding pure films. The current work reveals that the polymer preaggregation is the key reason enabling optimal morphology and high performance in N2200‐based all‐PSCs, and this strategy may be potentially applied in other systems to optimize the morphology and performance of all‐PSCs.

The polyelectrolyte‐doped SnO₂ film can efficiently improve the perovskite solar cell (PSC) performance and stability. Compared with the pristine SnO₂ film, the better energy level alignment, larger built‐in field, enhanced electron transfer/extraction, and reduced charge recombination all contribute to the improved device performance. Finally, a power conversion efficiency of 20.61% is successfully achieved for the PSC prepared under low temperature.

The charge transport layer is crucial to the performance and stability of the perovskite solar cells (PSCs). Compared with other conventional metal oxide electron transport materials, SnO₂ has a deeper conduction band and higher electron mobility, and can efficiently serve as an electron transport layer to facilitate charge extraction and transfer. Herein, an optimized low‐temperature solution‐processed SnO₂ electron transport layer is achieved by doping polyethylenimine polyelectrolyte into SnO₂ for the first time in the PSCs. It is found that the performance of all aspects of the doped SnO₂ film is improved over that of the pristine SnO₂ film. The better energy level alignment, larger built‐in field, enhanced electron transfer/extraction, and reduced charge recombination all contribute to the improved device performance. Finally, a PSC with a power conversion efficiency of 20.61% is successfully prepared under low temperature below 150 °C. Moreover, the stability of the doped SnO₂‐based device is also greatly improved.

The aprotic butyldimethylsulfonium‐driven MAPbI₃ perovskite shows a much more pronounced effect on the improvement of moisture stability compared to the protic butylammonium (BA)‐based counterpart. The BA having a potential hydrogen donor, which exists on the surface and/or grain boundaries, is vulnerable to H₂O‐induced degradation initiators, resulting in the faster hydration followed by the irreversible degradation of perovskites.

Abstract

Many organic cations in halide perovskites have been studied for their application in perovskite solar cells (PSCs). Most organic cations in PSCs are based on the protic nitrogen cores, which are susceptible to deprotonation. Here, a new candidate of fully alkylated sulfonium cation (butyldimethylsulfonium; BDMS) is designed and successfully assembled into PSCs with the aim of increasing humidity stability. The BDMS‐based perovskites retain the structural and optical features of pristine perovskite, which results in the comparable photovoltaic performance. However, the fully alkylated aprotic nature of BDMS shows a much more pronounced effect on the increase in humidity stability, which emphasizes a generic electronic difference between protic ammonium and aprotic sulfonium cation. The current results would pave a new way to explore cations for the development of promising PSCs.

This study investigates ratchets driven by two sets of asymmetrically spaced interdigitated finger electrodes situated within a dielectric layer on top of an indium–gallium–zinc oxide field‐effect transistor. The maximum experimentally observed efficiency is above 10% at 5 MHz. Via simulations, engineering guidelines are established for increasing the power output and efficiency into the THz range and beyond.

Abstract

Electronic ratchets use a periodic potential with broken inversion symmetry to rectify undirected (electromagnetic, EM) forces and can in principle be a complement to conventional diode‐based designs. Unfortunately, ratchet devices reported to date have low or undetermined power conversion efficiencies, hampering applicability. Combining experiments and numerical modeling, field‐effect transistor‐based ratchets are investigated in which the driving signal is coupled into the accumulation layer via interdigitated finger electrodes that are capacitively coupled to the field effect transistor channel region. The output current–voltage curves of these ratchets can have a fill factor >> 0.25 which is highly favorable for the power output. Experimentally, a maximum power conversion efficiency well over 10% at 5 MHz, which is the highest reported value for an electronic ratchet, is determined. Device simulations indicate this number can be increased further by increasing the device asymmetry. A scaling analysis shows that the frequency range of optimal performance can be scaled to the THz regime, and possibly beyond, while adhering to technologically realistic parameters. Concomitantly, the power output density increases from ≈4 W m⁻² to ≈1 MW m⁻². Hence, this type of ratchet device can rectify high‐frequency EM fields at reasonable efficiencies, potentially paving the way for actual use as energy harvester.

Bright light! Doping very low amounts of Mn²⁺ in (TDMP)PbBr₄ was carried out to achieve a bright pure white emission with very high efficiency (PLQY=60 %) and ultra‐high color rendering (CRI=96).

Abstract

Near‐UV‐pumped white‐light‐emitting diodes with ultra‐high color rendering and decreased blue‐light emission is highly desirable. However, discovering a single‐phase white light emitter with such characteristics remains challenging. Herein, we demonstrate that Mn doping as low as 0.027 % in the hybrid post‐perovskite type (TDMP)PbBr₄ (TDMP=trans‐2,5‐dimethylpiperaziniium) enables to achieve a bright pure white emission replicating the spectrum of the sun's rays. Thus, a white phosphor exhibiting an emission with CIE coordinates (0.330, 0.365), a high photoluminescence quantum yield of 60 % (new record for white light emission of hybrid lead halides), and an ultra‐high color rendering index (CRI=96, R9=91.8), corresponding to the record value for a single phase emitter was obtained. The investigation of the photoluminescence properties revealed how free excitons, self‐trapped excitons, and low amount of Mn dopants are coupled to give rise to such pure white emission.

Publication date: December 2019

Source: Nano Energy, Volume 66

Author(s): Xiaolan Liu, Ke Cheng, Peng Cui, Hui Qi, Huaifang Qin, Guangqin Gu, Wanyu Shang, Shujie Wang, Gang Cheng, Zuliang Du

Graphical abstract

A hybrid energy harvester has been fabricated to harvest the energy of sunlight and raindrops. A bi-functional nano-wrinkled PDMS film is introduced onto the surface of the solar cell that could serve as both a triboelectric layer to collect the raindrop energy and an anti-reflective layer of the solar cell to enhance the solar energy conversion efficiency. The methodology could be easily transferred to all other type of solar cells without any barrier, showing great universality and broad application prospects.

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Haoran Wang, Yepin Zhao, Zhenyu Wang, Yunfei Liu, Zipeng Zhao, Guangwei Xu, Tae-Hee Han, Jin-Wook Lee, Chen Chen, Daqian Bao, Yu Huang, Yu Duan, Yang Yang

Graphical abstract

Publication date: March 2020

Source: Nano Energy, Volume 69

Author(s): Peng Chen, Zhiliang Wang, Songcan Wang, Miaoqiang Lyu, Mengmeng Hao, Mehri Ghasemi, Mu Xiao, Jung-Ho Yun, Yang Bai, Lianzhou Wang

Graphical abstract

Luminescent europium-doped titania (Eu-TiO₂) thin films are fabricated via a facile and scalable chemical-bath deposition at low-temperature (70 °C). The use of Eu-TiO₂ in planar perovskite solar cells (PSCs) enables effective down-shifting of damaging UV light to extra visible luminescence and also leads to a more optimal band alignment, resulting in an enhanced power conversion efficiency of 21.4% and significantly improved device stability under UV illumination for 500 h.

Crystallizing perovskites on an ionic liquid‐modified SnO₂ substrate causes a shift of the perovskite Fermi level toward the conduction band and decreases the density of trap states in the perovskite. This results in a reduction of nonradiative recombination losses and, consequently, improved solar cell efficiencies.

Abstract

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge‐selective layers being where most recombination losses occur. In this study, a fluorinated ionic liquid (IL) is employed to modify the perovskite/SnO₂ interface. Using Kelvin probe and photoelectron spectroscopy measurements, it is shown that depositing the perovskite onto an IL‐treated substrate results in the crystallization of a perovskite film which has a more n‐type character, evidenced by a decrease of the work function and a shift of the Fermi level toward the conduction band. Photoluminescence spectroscopy and time‐resolved microwave conductivity are used to investigate the optoelectronic properties of the perovskite grown on neat and IL‐modified surfaces and it is found that the modified substrate yields a perovskite film which exhibits an order of magnitude lower trap density than the control. When incorporated into solar cells, this interface modification results in a reduction in the current–voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady‐state PCEs exceeding 20%.

Publication date: 15 January 2020

Source: Joule, Volume 4, Issue 1

Author(s): Jiahuan Zhang, Zaiwei Wang, Aditya Mishra, Maolin Yu, Mona Shasti, Wolfgang Tress, Dominik Józef Kubicki, Claudia Esther Avalos, Haizhou Lu, Yuhang Liu, Brian Irving Carlsen, Anand Agarwalla, Zishuai Wang, Wanchun Xiang, Lyndon Emsley, Zhuhua Zhang, Michael Grätzel, Wanlin Guo, Anders Hagfeldt

The aprotic butyldimethylsulfonium‐driven MAPbI₃ perovskite shows a much more pronounced effect on the improvement of moisture stability compared to the protic butylammonium (BA)‐based counterpart. The BA having a potential hydrogen donor, which exists on the surface and/or grain boundaries, is vulnerable to H₂O‐induced degradation initiators, resulting in the faster hydration followed by the irreversible degradation of perovskites.

Abstract

Many organic cations in halide perovskites have been studied for their application in perovskite solar cells (PSCs). Most organic cations in PSCs are based on the protic nitrogen cores, which are susceptible to deprotonation. Here, a new candidate of fully alkylated sulfonium cation (butyldimethylsulfonium; BDMS) is designed and successfully assembled into PSCs with the aim of increasing humidity stability. The BDMS‐based perovskites retain the structural and optical features of pristine perovskite, which results in the comparable photovoltaic performance. However, the fully alkylated aprotic nature of BDMS shows a much more pronounced effect on the increase in humidity stability, which emphasizes a generic electronic difference between protic ammonium and aprotic sulfonium cation. The current results would pave a new way to explore cations for the development of promising PSCs.

Vacuum‐assisted growth control (VAGC) allows growing micrometer‐sized and pinhole‐free low‐bandgap (E _G ≈1.27 eV) perovskite thin films. VAGC exhibits promising reproducibility and potential in fabrication of larger active‐area solar cells up to 1 cm². The efficient low‐bandgap perovskite solar cells fabricated by VAGC enable efficient four‐terminal all‐perovskite tandem solar cells with power conversion efficiencies up to 23%.

Abstract

All‐perovskite multijunction photovoltaics, combining a wide‐bandgap (WBG) perovskite top solar cell (E _G ≈1.6–1.8 eV) with a low‐bandgap (LBG) perovskite bottom solar cell (E _G < 1.3 eV), promise power conversion efficiencies (PCEs) >33%. While the research on WBG perovskite solar cells has advanced rapidly over the past decade, LBG perovskite solar cells lack PCE as well as stability. In this work, vacuum‐assisted growth control (VAGC) of solution‐processed LBG perovskite thin films based on mixed Sn–Pb perovskite compositions is reported. The reported perovskite thin films processed by VAGC exhibit large columnar crystals. Compared to the well‐established processing of LBG perovskites via antisolvent deposition, the VAGC approach results in a significantly enhanced charge‐carrier lifetime. The improved optoelectronic characteristics enable high‐performance LBG perovskite solar cells (1.27 eV) with PCEs up to 18.2% as well as very efficient four‐terminal all‐perovskite tandem solar cells with PCEs up to 23%. Moreover, VAGC leads to promising reproducibility and potential in the fabrication of larger active‐area solar cells up to 1 cm².

After the substitution of F atom on the benzothiadiazole (BT) acceptor unit of the polymer backbone, the power conversion efficiency of the polymer solar cell improves significantly due to the more balanced charge transport and small charge extraction time.

The fast evolution of the narrow bandgap nonfullerene acceptors requires new conjugated wide bandgap polymers for the use of nonfullerene polymer solar cells (PSCs). Herein, two new wide bandgap A1–D1–A2–D1 conjugated polymers with the same dithieno[2,3‐e:3′,2′‐g]isoindole‐7,9(8H)‐dione (DTID) acceptor (A1) and thiophene donor (D1) and different A2 acceptor units, i.e., benzothiadiazole (BT) and fluorinated benzothiadiazole (f‐BT) denoted as P113 and P114, are designed, and the effect of fluorination of the BT acceptor unit on the photovoltaic properties of PSCs using nonfullerene acceptors is investigated. It is found that the incorporation of fluorine atom into the BT acceptor unit increases the absorption coefficients and the relative dielectric constant. The increase in photoluminescence quenching, reduction in charge recombination loss, and improvement in the charge carrier life are observed for P114. All these factors result in the drastically improved power conversion efficiency of P114:ITIC‐m‐based PSC to 10.42% with a small energy loss of 0.56 eV as compared with the P113 counterpart (8.74% with an energy loss of 0.69 eV) under identical conditions. The low energy loss is beneficial to overcome the trade‐off between open‐circuit voltage and short‐circuit current.

An efficient solution‐processed interconnecting layer is designed for nonfullerene tandem solar cells by introducing hydrogen molybdenum bronze (H _x MoO₃). H _x MoO₃ circumvents wetting issues of aqueous PEDOT:PSS on the hydrophobic surface of active layers. Electrically, H _x MoO₃ films have high work function and improve hole collection. The H _x MoO₃ delivers high‐performance inverted single‐junction and tandem nonfullerene solar cells.

Tandem solar cells are attractive because they can break the Shockley–Queisser efficiency limit of single‐junction cells. However, it is still quite challenging to fabricate efficient nonfullerene tandem organic solar cells (OSCs) because of their complicated and vulnerable multilayers and interfaces. The interconnecting layer (ICL) between two subcells is the key part in efficient tandem solar cells, which should be designed properly based on the property of active layers. Herein, the incorporation of hydrogen molybdenum bronze (H _x MoO₃) between the bottom active layer and PEDOT:PSS/ZnO to form a new ICL is proposed. The contribution of the H _x MoO₃ is two fold: 1) it can well wet the hydrophobic active layer surface and form a uniform film. It provides a hydrophilic surface for the following deposition of PEDOT:PSS; 2) the H _x MoO₃ has a high work function of 5.4 eV that is higher than PEDOT:PSS (5.0 eV) and can extract holes more efficiently from the active layer with the deep highest occupied molecular orbital energy level. Nonfullerene tandem solar cells with a high fill factor of 76.0% and power conversion efficiency of 15.03% are obtained with this ICL.

The origin of the weak conductivity of Sb₂S₃ is unraveled. Improving the conductivity through direct extrinsic doping is intrinsically confined due to the comparable high concentration of intrinsic donor and acceptor. Interestingly, O doping fills the dominant donor V_S2 and makes the p‐type doping feasible. Therefore, a new strategy for overcoming the efficiency bottleneck of Sb₂S₃ solar cells is proposed.

The photovoltaic efficiency increase in Sb₂S₃‐based solar cells has stagnated for 5 years since the highest efficiency of 7.5% was achieved in 2014. One important bottleneck is the high electrical resistivity of Sb₂S₃. The first‐principle calculations reveal that the high‐resistivity results from the compensation between the intrinsic donor V_S and acceptors V_Sb, Sb_S, and S_Sb which have comparably high concentration (low formation energy). The compensation also limits the improvement of conductivity through direct extrinsic doping. Further calculations of O dopants show that O_S has low formation energy, so the dominant intrinsic donor V_S can be passivated by O and thus the p‐type doping limit imposed by V_S can be overcome. Meanwhile, other p‐type limiting and recombination‐center donor defects can be suppressed under the S‐rich condition, which explains why the highest efficiency is achieved in O‐doped Sb₂S₃ after sulfurization. Given the unexpected beneficial effects of O doping and sulfurization, a two‐step doping strategy is proposed for overcoming the efficiency bottleneck: 1) use O to passivate the V_S and S‐rich condition to suppress other detrimental defects, making p‐type doping feasible and minority carrier lifetime long; 2) introduce other p‐type dopants to increase hole carrier concentration.

Bulky cation addition to perovskite solar cells is demonstrated as an effective means of significantly improving device performance. Detailed structural characterization reveal additives are located at surfaces and grain boundaries, resulting in suppression of nonradiative recombination. Judicious cation selection results in MAPI‐based perovskite cells with a power conversion efficiency >20% and MAPBrI cells with a V _oc of 1.22 V.

Abstract

The origin of performance enhancements in p‐i‐n perovskite solar cells (PSCs) when incorporating low concentrations of the bulky cation 1‐naphthylmethylamine (NMA) are discussed. A 0.25 vol % addition of NMA increases the open circuit voltage (V _oc) of methylammonium lead iodide (MAPbI₃) PSCs from 1.06 to 1.16 V and their power conversion efficiency (PCE) from 18.7% to 20.1%. X‐ray photoelectron spectroscopy and low energy ion scattering data show NMA is located at grain surfaces, not the bulk. Scanning electron microscopy shows combining NMA addition with solvent assisted annealing creates large grains that span the active layer. Steady state and transient photoluminescence data show NMA suppresses non‐radiative recombination resulting from charge trapping, consistent with passivation of grain surfaces. Increasing the NMA concentration reduces device short‐circuit current density and PCE, also suppressing photoluminescence quenching at charge transport layers. Both V _oc and PCE enhancements are observed when bulky cations (phenyl(ethyl/methyl)ammonium) are incorporated, but not smaller cations (Cs/MA)—indicating size is a key parameter. Finally, it demonstrates that NMA also enhances mixed iodide/bromide wide bandgap PSCs (V _oc of 1.22 V with a 1.68 eV bandgap). The results demonstrate a facile approach to maximizing V _oc and provide insights into morphological control and charge carrier dynamics induced by bulky cations in PSCs.

Publication date: 15 January 2020

Source: Joule, Volume 4, Issue 1

Author(s): Kangmin Lee, Namwoo Kim, Kwangjin Kim, Han-Don Um, Wonjoo Jin, Deokjae Choi, Jeonghwan Park, Kyung Jin Park, Seungwoo Lee, Kwanyong Seo

Spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra, is employed to study degradation across perovskite and perovskite/silicon tandem solar cells. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap and intensity.

Abstract

Instability in perovskite solar cells is the main challenge for the commercialization of this solar technology. Here, a contactless, nondestructive approach is reported to study degradation across perovskite and perovskite/silicon tandem solar cells. The technique employs spectrally and spatially resolved absorptivity at sub‐bandgap wavelengths of perovskite materials, extracted from their luminescence spectra. Parasitic absorption in other layers, carrier diffusion, and photon smearing phenomena are all demonstrated to have negligible effects on the extracted absorptivity. The absorptivity is demonstrated to reflect real degradation in the perovskite film and is much more robust and sensitive than its luminescence spectral peak position, representing its optical bandgap, and intensity. The technique is applied to study various common factors which induce and accelerate degradation in perovskite solar cells including air and heat exposure and light soaking. Finally, the technique is employed to extract the individual absorptivity component from the perovskite layer in a monolithic perovskite/silicon tandem structure. The results demonstrate the value of this approach for monitoring degradation mechanisms in perovskite and perovskite/silicon tandem cells at early stages of degradation and various fabrication stages.

A near‐infrared (NIR)‐harvesting perovskite solar cell with a power‐conversion efficiency of 21.6% and an operational half‐life of 1900 h is achieved by directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates defects in the perovskite active layer.

Abstract

Typical lead‐based perovskites solar cells show an onset of photogeneration around 800 nm, leaving plenty of spectral loss in the near‐infrared (NIR). Extending light absorption beyond 800 nm into the NIR should increase photocurrent generation and further improve photovoltaic efficiency of perovskite solar cells (PSCs). Here, a simple and facile approach is reported to incorporate a NIR‐chromophore that is also a Lewis‐base into perovskite absorbers to broaden their photoresponse and increase their photovoltaic efficiency. Compared with pristine PSCs without such an organic chromophore, these solar cells generate photocurrent in the NIR beyond the band edge of the perovskite active layer alone. Given the Lewis‐basic nature of the organic semiconductor, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination‐induced ion migration. As a consequence, perovskite solar cells with organic chromophore exhibit an enhanced efficiency of 21.6%, and substantively improved operational stability under continuous one‐sun illumination. The results demonstrate the potential generalizability of directly incorporating a multifunctional organic semiconductor that both extends light absorption and passivates surface traps in perovskite active layers to yield highly efficient and stable NIR‐harvesting PSCs.

High‐efficiency organic solar cells are achieved through the use of a new electron acceptor AQx‐2 with a quinoxaline‐containing fused core. The increase in performance is attributed to the optimized phase separation morphology that significantly boosts hole transfer and suppresses geminate recombination. The power conversion efficiency of these devices, 16.4%, is the highest certified value for binary organic solar cells.

Abstract

Manipulating charge generation in a broad spectral region has proved to be crucial for nonfullerene‐electron‐acceptor‐based organic solar cells (OSCs). 16.64% high efficiency binary OSCs are achieved through the use of a novel electron acceptor AQx‐2 with quinoxaline‐containing fused core and PBDB‐TF as donor. The significant increase in photovoltaic performance of AQx‐2 based devices is obtained merely by a subtle tailoring in molecular structure of its analogue AQx‐1. Combining the detailed morphology and transient absorption spectroscopy analyses, a good structure–morphology–property relationship is established. The stronger π–π interaction results in efficient electron hopping and balanced electron and hole mobilities attributed to good charge transport. Moreover, the reduced phase separation morphology of AQx‐2‐based bulk heterojunction blend boosts hole transfer and suppresses geminate recombination. Such success in molecule design and precise morphology optimization may lead to next‐generation high‐performance OSCs.